m08#27

FN · **Posted:** Sun Nov 02, 2008 2:48 pm

Comet A is seen near the Earth every 12 years while comet B every 20 years. If both comets were observed in 1979, for how many years do we have to wait to see the two comets together again? (Assume now is 2004)

(A) 16
(B) 20
(C) 25
(D) 32
(E) 35

[Reveal] Spoiler: OA

E

Source: GMAT Club Tests - hardest GMAT questions

Again the wording is very bad, are you asking in how many years does it take both of them to appear that is 60 years, however if you want to know, how long from NOW do we have to wait to see the comets reappear together?? is better wording

ameyaberi · **Posted:** Thu Aug 26, 2010 8:53 am

Hi, the corrent answer is E ie after 35 years.
Comet A is seen after 12 years and B after 20 years.
So they will be seen together after LCM(12,20) = 60 years.
After every 60 years they will be seen together.
They were seen together in 1979, so 60 years after 1979 ie 2039.
So number of years 2039-2004(2004 is current year) = 35 years.

For people who dont know what LCM stands for it is Least Common Multiple.
LCM of two numbers can be found out as below:
12 = 2*2*3 (Find the prime factors)
20 = 2*2*5 (Find the prime factors)
LCM(12, 20) = Common * Uncommon = 2*2*3*5 = 60

Send me kudos if you like my post.

FN · **Posted:** Tue Nov 04, 2008 8:36 am

GMAT TIGER wrote:

fresinha12 wrote:

Comet A is seen near the Earth every 12 years while comet B every 20 years. If both comets were observed in 1979, for how many years do we have to wait to see the two comets together again? (Assume now is 2004)

* 16
* 20
* 25
* 32
* 35

Again the wording is very bad, are you asking in how many years does it take both of them to appear that is 60 years, however if you want to know, how long from NOW do we have to wait to see the comets reappear together?? is better wording

C. 25

NOPE..

so it takes 60 years..so from 1979--60 years is 2039..so when we see it next will be 2039-2004 or 35 yrs from now..

Jdam · **Posted:** Tue Sep 06, 2011 12:56 pm

ameyaberi wrote:

Hi, the corrent answer is E ie after 35 years.
Comet A is seen after 12 years and B after 20 years.
So they will be seen together after LCM(12,20) = 60 years.
After every 60 years they will be seen together.
They were seen together in 1979, so 60 years after 1979 ie 2039.
So number of years 2039-2004(2004 is current year) = 35 years.

For people who dont know what LCM stands for it is Least Common Multiple.
LCM of two numbers can be found out as below:
12 = 2*2*3 (Find the prime factors)
20 = 2*2*5 (Find the prime factors)
LCM(12, 20) = Common * Uncommon = 2*2*3*5 = 60

Send me kudos if you like my post.

Kudos.

This is how I calculated mine although I don't think I would be able to explain it as well as you.

GMAT TIGER · **Posted:** Sun Nov 02, 2008 3:39 pm

fresinha12 wrote:

Comet A is seen near the Earth every 12 years while comet B every 20 years. If both comets were observed in 1979, for how many years do we have to wait to see the two comets together again? (Assume now is 2004)

* 16
* 20
* 25
* 32
* 35

Again the wording is very bad, are you asking in how many years does it take both of them to appear that is 60 years, however if you want to know, how long from NOW do we have to wait to see the comets reappear together?? is better wording

C. 25

dzyubam · **Posted:** Tue Nov 04, 2008 10:21 am

Thanks a lot. +1. We'll edit the wording of this one.

fresinha12 wrote:

Comet A is seen near the Earth every 12 years while comet B every 20 years. If both comets were observed in 1979, for how many years do we have to wait to see the two comets together again? (Assume now is 2004)

* 16
* 20
* 25
* 32
* 35

Again the wording is very bad, are you asking in how many years does it take both of them to appear that is 60 years, however if you want to know, how long from NOW do we have to wait to see the comets reappear together?? is better wording

pclg · **Posted:** Thu Aug 26, 2010 6:52 am

So... I get that 2039 - 1979 equals 35 but how do you recognize the multiplication of 3 * 2 * 2* 5 to give you the years.

I did separate the 12 and 20 yrs in Greatest Common Factor and Least Common Multiple (4)*(15) but I don´t understand why the multiplication of both will give me the answer of when both comets will see each other again.

Please explain!

resh · **Posted:** Thu Aug 26, 2010 8:26 am

Can u plz explain the correct ans.?

aesopturtle · **Posted:** Thu Aug 26, 2010 9:28 am

Hi, how do you that this kind of question needs to be solved with LCM?
I would like a reasonable explanation if possible please.

lnarayanan · **Posted:** Thu Aug 26, 2010 11:30 am

Comet A is seen near the Earth every 12 years while comet B every 20 years. If both comets were observed in 1979, for how many years do we have to wait to see the two comets together again? (Assume now is 2004)

Comet " B " will appear once in 20 years...
20 * 1 = 20th year
20 * 2 = 40th year
20 * 3 = 60th year

Comet " A " will appear once in 12 years..
12 * 1 = 12th year
12 * 2 = 24th year
12 * 3 = 36th year
12 * 4 = 48th year
12 * 5 = 60th year

When we compare above table, Both the comet will appear again in 60th year from 1979
= 1979+60 =2039th year
current year is 2004
therefore 2039-2004 = 35 years..

kjs · **Posted:** Thu Aug 26, 2010 11:42 am

+1 for E: 35

amneetpadda · **Posted:** Thu Aug 26, 2010 5:00 pm

Answer is E

LCM of 12 and 20 is 60 hence they would meet in 2029.i.e. 2029-2004 = 35

Why LCM......Lcm can also be explained as occurrence of different digits after some interval so that the result is same.

mission2009 · **Posted:** Fri Aug 27, 2010 4:15 am

Ans:E,
LCM of 12 & 20.

One query:
Do we get such easy questions in GMAT?

rohinipathi · **Posted:** Fri Aug 27, 2010 9:16 pm

Its E

12 - 20
24 - 40
36 - 60
48 - 80
60 - 100

Its at 60 years, both of them appear together..
1979 + 60 yr = 2039
2039 - 2004 = 35 years

zisis · **Posted:** Mon Aug 30, 2010 9:13 am

FN wrote:

Comet A is seen near the Earth every 12 years while comet B every 20 years. If both comets were observed in 1979, for how many years do we have to wait to see the two comets together again? (Assume now is 2004)

(A) 16
(B) 20
(C) 25
(D) 32
(E) 35

[Reveal] Spoiler: OA

E

Source: GMAT Club Tests - hardest GMAT questions

Again the wording is very bad, are you asking in how many years does it take both of them to appear that is 60 years, however if you want to know, how long from NOW do we have to wait to see the comets reappear together?? is better wording

LCM is 60, 1979+60 = 2039-2004 = 35, IMO E

prashantbacchewar · **Posted:** Tue Aug 31, 2010 5:59 am

LCM of 12 and 20 is 60. Hence 60 years after 1979 these will be seen again.
Now it is 2004. So it is alrady 25 years since 1979. So additional 60-25 = 35 Years will be needed.
Hence "E"

alexgmd · **Posted:** Fri Sep 02, 2011 4:26 am

E. easy one

jagdeepsingh1983 · **Posted:** Fri Sep 02, 2011 7:29 am

E, When both appear back must have total years divisible by 12 & 20, what is multiple of 20 divisible by 12 is 60

1979+60= 2039
Today is 2004 to years to come 2039-2004 = 35 years

petrifiedbutstanding · **Posted:** Sat Sep 03, 2011 7:54 am

Personally, I think the quickest way to solve this problem is to find a common multiple of 12 and 20, viz., 60.

Add 60 years to 1979, viz., 2039. 2039 - 2004 = 35 years.

Should pace under 40 secs this way.

bidmanager · **Posted:** Tue Sep 06, 2011 3:39 pm

ameyaberi wrote:

Hi, the corrent answer is E ie after 35 years.
Comet A is seen after 12 years and B after 20 years.
So they will be seen together after LCM(12,20) = 60 years.
After every 60 years they will be seen together.
They were seen together in 1979, so 60 years after 1979 ie 2039.
So number of years 2039-2004(2004 is current year) = 35 years.

For people who dont know what LCM stands for it is Least Common Multiple.
LCM of two numbers can be found out as below:
12 = 2*2*3 (Find the prime factors)
20 = 2*2*5 (Find the prime factors)
LCM(12, 20) = Common * Uncommon = 2*2*3*5 = 60

Send me kudos if you like my post.

I got E as well. But, it took a while to arrive at the answer.

I know LCM, but didn't know that we can use it for this problem. Can you please explain any other problem scnerios where LCM can used to solve a problem.

Waiting for the day when I will be able to approach and solve maths the way you guys do it! truly commendable!

m08#27

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m08#27

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